ABSTRACT

We report the occurrence of glimmerite and melteigite xenoliths from a hybrid Early Cretaceous camptonite - tinguaite dyke from the Nongchram fault zone, East Garo Hills, Shillong Plateau, North East India. Glimmerite xenolith contains predominantly biotite with subordinate to minor amounts of pyroxene, ferro- dolomite, rutile, and ilmenite. The melteigite xenolith is dominated by pyroxene and contain pseudo-nepheline (altered to analcime) and rutile. The host campto-tinguaite dyke features a distinctive porphyritic-panidiomorphic and a tinguaitic texture with clinopyroxenes as macrocrysts, microcrysts and clots and amphibole as phenocrysts with accessory minerals such as apatite, magnetite, rutile, and ilmenite. In-situ trace element geochemistry of pyroxenes from the xenoliths and the host rock reveals significant enrichment in LILEs such as Ba and Sr, as well as LREEs like La and Ce, suggesting an enriched mantle source. The geothermobarometric data for pyroxenes from the (i) glimmerite and melteigite xenoliths and macrocrysts and (ii) microcrysts and clots from the campto-tinguaite suggest varying crystallization pressures and temperatures, indicating different depths of origin ranging from 23 - 96 km. The mineral composition, in-situ trace element data of pyroxenes as well as the geothermobarometric study of clinopyroxenes from both the host and the xenoliths, suggest involvement of multiple shallower magma chambers composed of the camptonitic and tinguaitic magmas generated during distinct pulses, thereby forming a complex magmatic plumbing system. The presence of orogenic geochemical signatures in the minerals of xenoliths and anorogenic geochemical signatures in the host campto-tinguaite pyroxenes imply a complex tectono-magmatic setting, with contributions from both the plume and subduction-modified mantle sources.